OBJECTIVE To identify the nucleus retroambiguus (NRA) and to determine its projections to the lumbosacral cord. The nucleus retroambiguus (NRA) consists of premotor neurons in the caudal medulla. It is involved in expiration, vomiting, vocalization, and probably reproductive behavior via projections to distinct motoneuronal cell groups. Since no information is available about the NRA and its efferent pathways in primates, the present study examines NRA projections to the lumbosacral cord in female rhesus monkeys. To identify the NRA, wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) was injected into the lumbosacral cord in three monkeys. To study the distribution of NRA axons in the lumbosacral cord, WGA-HRP injections were made into the NRA in six monkeys. These six monkeys also received cholera toxin subunit b (CTb) injections into different hindlimb, axial, and pelvic floor muscles to identify motoneuronal cell groups receiving input from the NRA. The results show that NRA neurons projecting to the lumbosacral cord are mainly located between 1 to 4 mm caudal to the obex, and project stro ngly to external oblique and pelvic floor motoneurons, whereas projections to iliopsoas and axial motoneurons are weaker. At the ultrastructural level, NRA-terminal profiles make asymmetrical contacts with dendrites in these motoneuronal cell groups, and contain large amounts of spherical and a few dense core vesicles. It can be concluded that the NRA is well developed in the monkey, and that there exists a direct pathway from the NRA to lumbosacral motoneurons in the rhesus monkey. The finding that the NRA projects to a partly different set of motoneuronal cell groups, as compared to other species, fits the concept that the NRA is not only involved in expiration related activities, but also in species specific receptive and submissive behaviors. FUTURE DIRECTIONS We will determine the effects of estrogen on monosynaptic projections from the nucleus retroambiguus. KEY WORDS nucleus retroambiguus, lumbosacral cord, electron microscopy, neuronal tracing, sexual behavior, respiration FUNDING Supplemental funding to RR00167 PUBLICATIONS Van der Horst, V.G.J.M., Terasawa, E., Holstege, G., Ralston III, H.J. Monosynaptic projections from the nucleus retroambiguus to distinct groups of lumbosacral motoneurons in the female rhesus monkey. Abstracts of the 28th Annual Meeting of the Society for Neuroscience 24 950 (No. 372.17), 1998. ]A]